We are using a 250 infared heat lamp and found we had to place it much higher initially than we thought we would.
Also, keep in mind those lamps are designed to heat differently than an incandescent. For example, our thermometer (wire wound outdoor type) records a much higher temp from the constant heat of the lamp.
For us, our best bet was to provide more than enough room (based on Storey's space requirements) and let the birds decide on how they felt.
I still turn the lamp on at night and anytime the ambient temp is below 70 degrees in the coop.
If it's going to drop below 50 at night, I lower the lamp to somewhere between 36 and 40 inches above the coop floor. Several of our chicks (28 days old today) are already preferring to roost, so they don't even get the benefit of the lamp.
Last night, with an expect low of nearly 60, I only turned the lamp on and left it 6 feet above the ground. This morning all the chicks were happy, fluttering around, and ready for breakfast. From the looks of the roost... they must have had quite a slumber party.
I have a lot more details of our temperature trials and brooder/coop pics in my
blog, which will be updated again today.